Conventionally, a multi-fuel internal combustion engine has been developed, which can be operated by mixedly combusting a plurality of types of fuels such as liquid fuels (for example, diesel oil, gasoline or the like) and gas fuels for example, compressed natural gas, hydrogen gas or the like). For example, Patent Document 1 discloses an auxiliary fuel ignition type gas engine in which diesel oil is used as an auxiliary fuel. In this auxiliary fuel ignition type gas engine, diesel oil is used as an ignition source, and diesel oil and natural gas are mixedly combusted.